The present invention relates to a highly-sensitive quadrupole mass spectrometer which detects an ion current by an AC-modulation system.
In the prior art, a mass spectrometer ionizes gaseous molecules by electron bombardment, and the thus-generated ions are classified and detected electrically and magnetically in accordance with their masses. Since such a mass spectrometer has a sensitivity of about 3.times.10.sup.-3 A/Torr, the ion currents below 10.sup.-12 A are found to be very weak, when measured at a partial pressure of a very high vacuum of less than 10.sup.-8 Torr. When detecting such weak DC currents, accurate measurement is difficult because of the occurrence of offset and drift, and the generation of noise. Sensitivity can be improved by the use of a secondary electron multiplier, but a high-voltage source is necessary therefor, which has the disadvantage of increasing the size of the system.